1. Field of the invention
This invention pertains to a velocity change sensor or accelerometer used in motor vehicles for detecting sudden changes in velocity and for activating in response a passenger restraining device such as an air bag. More particularly, this device includes an element which moves to a preset position in response to a sudden deceleration to close pair of contact blades, said device also having a spring for biasing the element.
2. Background of the Invention
Studies have been made which indicate that injuries in motor vehicle accidents, especially at high speeds, can be substantially reduced or eliminated by the use of passenger restraint systems. (The term "passenger" is used to cover the driver of a car as well.) These systems include an inflatable balloon usually termed an air bag which normally is stored away in the instrument panel or the steering wheel. When the motor vehicle is subjected to a sudden deceleration, the air bag is inflated and is deployed automatically in a position which cushions the passengers, restrains their movement and prevents contact between them and the automobile interior such as the windshield, the steering wheel. The instrument panel and so on. Of course, a crucial element of all such systems is the velocity change sensor or accelerometer which initiates the inflation and deployment of the air bags. The motion of the motor vehicle must be carefully and precisely monitored so that the air bags can be deploted very fast, before the passengers suffer any substantial injury.
A velocity change sensor is disclosed in U.S. Pat. No. 4,329,549 assigned to the same company as the present invention. This sensor comprises a tubular housing surrounding a metallic shell, a metal ball and a magnet biasing the ball toward a first end of the shell. At the second end of the shell there are a pair of electrical contact blades. The sensor is positioned in the motor vehicle in an orientation such that when the motor vehicle experiences a deceleration which exceeds a preset level, the ball moves from the first toward the second end, making contact with the two blades. Because the blades and the ball are made of an electrically conducting material, when the ball contacts the blades, an electrical path is established between the two blades. This electrical path is used to initiate a signal for the deployment of the air bags.
Because the contacts must have a certain inherent flexibility they are normally made of steel, copper or an alloy thereof, while the ball is usually gold plated to insure a good electrical contact between the ball and the contact blades. Because of the difference in the electrochemical activity between these materials, corrosion due to electrolysis may occur which may form an insulating layer on the balls and/or the contact blades, thus causing the sensor to malfunction. This problem is further aggravated if the chamber holding the ball and the contacts is open to the atmosphere because fumes and other contaminants from the automobile engine.
Another problem with the prior art sensor described above is that the biasing force on the ball is generated by a permanent magnet. Since the magnet has to generate a relatively strong magnetic field, it is large and waste space.